1. Field of the Invention
The present invention relates in general to digital receivers, and in particular to a method and apparatus for phase sampling using amplitude bits for Global Positioning System (GPS) receivers.
2. Description of the Related Art
Digital receivers are typically very complex devices that locate weak signals in sometimes difficult signal acquisition environments. Currently, federal government mandates for and interest in producing digital receivers, specifically digital GPS receivers, that are compatible with cellular telephones stems from a new Federal Communications Commission (FCC) requirement that cellular telephones be locatable within 20 feet once an emergency call, such as a “911” call (also referred to as Enhanced 911 or “E911”) is placed by a given cellular telephone. Such position data assists police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need or have legal rights to determine the cellular telephone's position. Further, GPS data can be used by the cellular user for directions, location of other locations that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular user via internet maps or other GPS mapping techniques, etc. Such data can be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers.
Typically, digital receivers use a phase-sampling technique to detect zero crossings of the incoming signal, and attempt to align or correlate these zero crossings with a local oscillator or crystal. This allows the receiver to “lock” onto the received signal and use the signal for tracking, position determination, and other information. However, the use of phase information alone makes the correlation between the incoming signal and the reference signal difficult and more time-consuming, which could leave an E911 caller in a dangerous situation longer than necessary. Further, the typical phase-sampling technique requires significant power consumption for smaller, hand-held devices such as cellular telephones.
Further, digital receivers are sometimes used in environments where Continuous Wave (CW) jamming is occurring, such as battlefields, congested signal areas such as airports, etc. CW jamming will degrade the receiver performance of the digital receiver, sometimes to the point where the digital receiver can no longer receive signals with any reliability. This could be dangerous for E911 callers that were unfortunate enough to be stranded in a location that is within a CW jamming area, as well as being potentially deadly for soldiers that are in a CW jamming area.
It can be seen, then, that there is a need in the art for a method and apparatus for improved phase-sampling techniques that can reduce the complexity of the digital receiver circuitry. It can also be seen that there is a need in the art for reducing the power consumption for phase-sampling and correlating within the digital receiver. It can also be seen that there is a need in the art for integrating GPS and cellular devices in such a manner that reduces the time required to obtain position information. It can also be seen that there is a need in the art for a method and apparatus that can help overcome CW jamming problems for digital receivers.